<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .box {
        display: flex;
        flex: auto;
        height: 300px;
        border: 1px solid #000;
    }

    .box div {
        /* height: 100px; */
        background-color: pink;
    }

    .box div:nth-child(1) {
        width: 200px;
    }
</style>

<body>
    <div class="box">
        <div>
            1
        </div>
        <div>
            2
        </div>
        <div>
            3
        </div>
    </div>

</body>

</html>